Examples of the uses to which such a press may be put include: sludge treatment in sewage works; drying peat; dewatering fibrous or cellulose materials in paper-making installations; and extracting juice from fruit to vegetable pulp.
The invention relates more particularly to a filter band press having a pair of porous endless bands that are moved continuously and are disposed to receive and press between them the material which is to be dehydrated. Presses of this nature are described, for example, in French patent documents FR-A-2 066 083, FR-B-2 580 946, and FR-A-1 526 592.
Document FR-A-1 526 592 describes a fruit press in which the filter bands present two parallel strands which sandwich the material to be dehydrated, in particular pieces of fruit or vegetables. These two strands extend horizontally and are themselves clamped between two presser plates constituted by frames carrying thrust rollers. Each presser plate acts against the associated length of band via a moving endless belt.
The side of the belt that presses against the filter band has a series of grooves hollowed out therein for removing the liquid which is extracted by pressing the cake which is imprisoned between the two bands, which liquid passes through the pores in the adjacent filter band. Appropriate sealing means are provided along the longitudinal edges of the filter bands for keeping the material between them.
In a variant, the endless belt is constituted by a series of pressing shoes which are hinged to one another like a crawler track.
This kind of press generally gives satisfaction in common applications, in particular when pressing fruit, since low or moderate pressure (about 5 bars) suffices for obtaining the desired extraction.
However, it is not suitable for applications where it is necessary to subject the cake material to pressures which are high or very high, about 50 bars or more. If the pressure is very high, then problems arise with driving the bands and the cake because of the high friction which is developed between the stationary parts and the moving parts. There is also the question of premature damage and wear of the moving parts, in particular of the filter bands. In some applications it is desirable to make these bands from a fine texture substance, e.g. linen cloth, which does not necessarily have good mechanical characteristics, in particular high traction strength.
Unfortunately, the biological purification treatment methods that are used in sewage works are producing sludges that are more and more difficult to dehydrate by conventional means at satisfactory cost. In addition, international standards concerning waste are changing to require ever higher dry contents, generally higher than 30%, without significant addition of other substances such as lime or carbon-based bulking agents such that the needs for high performance dedhydrating presses operating at high or very high pressure are becoming felt more and more strongly on the market.
That is why one of the objects of the present invention is to provide a press of the above-mentioned type which is capable of operating at considerably higher pressures than those that have been obtainable in the past, but without major modification to this type of press.
Another object of the invention is to propose a press whose structure is simple and robust, in which the various components are protected from premature damage or wear.
Another object is to provide a press which is easy to use and which is capable of operating at high throughput.